1. Field of the Invention
The present invention relates generally to extensible and retractable masts, and more specifically to an electronic or computer control system for controlling erection of such a mast.
2. Description of the Prior Art
Extensible masts and towers of various types are known in the prior art. An example of one such type of extensible mast is shown in U.S. Pat. No. 4,625,475, EXTENSIBLE MAST, by McGinnis, which is herein incorporated by reference. Such patent shows the creation of an extensible mast by placing three flexible metal tapes edge-to-edge to form a triangular cross-sectioned member. Cables are wrapped around the mast in order to make it rigid. The extensible mast described in the McGinnis patent is extended from a central location by wrapping cable around the triangular member formed by unrolling three spools of flexible metal material so that they form a triangular cross-section.
Towers and masts generally use a plurality of cables attached from selected points of the mast to anchor points on the ground in order to provide horizontal support for the mast. These are generally referred to as guys or guy cables. Three or four anchor locations are typically provided at points spaced away from the base of the mast. These anchor points are preferably located in directions from the mast which are equally spaced around a circle. Each anchor position may be located at different distances from the base of the mast.
Location of anchor points for a fixed mast or tower must take several conditions into consideration. Improved horizontal support of the mast or tower is provided by spacing the anchor positions as far away from the mast as possible. However, various terrain restrictions and other requirements may require that some anchor positions be located closer to the mast than others. Also, the terrain may dictate that some anchor locations be located at significantly different elevations from the base of the mast and from each other.
For taller structures, it is usually desirable or necessary to have guy cables located at several points along the height of the structure. For example, guy cables could be attached to the tower at every 50 feet of height, so that a 200 foot tower would have four sets of guy cables. One cable from each height is typically run to a single anchor location, so that the 200 foot tower would have four guy cables attached to each anchor point. These additional cables attached along the height of the tower prevent both bending of the tower due to horizontal loads and divergence from the vertical axis, and are especially desirable for masts which have a minimum amount of horizontal structural support. The extensible mast described above falls in this catagory, and preferably has several sets of guy cables along its height for tall structures.
In extensible masts of the type described above, the guy cables must be attached as the mast is being erected, and must be deployed from the anchor points at a rate consistent with the rate at which the mast is being raised. If the various anchor points are located at different distances from the mast, and at different heights relative to the base of the mast, deploying the guy cables as the mast is raised can be a very difficult process.
It would therefore be desirable for an automatic controller to adjust the rate at which guy cables are paid out from, and taken up at, anchor points in order to support an extensible mast while it is being extended or retracted. It would be further desirable if such controller could automatically compensate for variations in anchor point placement.